Artificial graphite particles and method for manufacturing same, nonaqueous electrolyte secondary cell, negative electrode and method for manufacturing same, and lithium secondary cell

ABSTRACT

Artificial graphite particles, having a secondary particle structure in which a plurality of primary particles composed of graphite are clustered or bonded together, and having a layer structure in which the edge portion of the primary particles is bent in a polyhedral shape.

This is a National Phase Application in the United States ofInternational Patent Application No. PCT/JP02/00564, filed Jan. 25,2002, which claims priority on Japanese Patent Application No.P2001-017141 filed Jan. 25, 2001; Japanese Patent Application No.P2001-270099 filed Sep. 6, 2001; and Japanese Patent Application No.P2001-341754 filed Nov. 7, 2001, the entire disclosures of which areincorporated herein by reference.

TECHNICAL FIELD

This invention relates to artificial graphite particles and a method formanufacturing the same, to a nonaqueous electrolyte secondary cellnegative electrode and method for manufacturing the same, and to alithium secondary cell.

BACKGROUND ART

Graphite powders and so forth, such as natural graphite, artificialgraphite produced by the graphitization of coke, and artificial graphiteproduced by the graphitization of an organic polymer or pitch, have beenused as negative electrode active materials for conventional lithiumsecondary cells. Organic polymers are added as a binder to thesegraphite powders, organic solvents or water are added to create a paste,the surface of a copper foil collector is coated with this graphitepaste, and the paste is dried to remove the solvent and produce anegative electrode for use in a lithium secondary cell. For example, asdisclosed in Japanese Patent Publication No. S62-23433, the problem ofinternal shorting caused by dendritic precipitation of lithium is solvedand the cycling characteristics are improved by using graphite for thenegative electrode active material. However, although a lithiumsecondary cell in which graphite is used for the negative electrode doeshave better cycling characteristics than a lithium secondary cell inwhich metallic lithium or a lithium alloy is used for the negativeelectrode, the following two problems remain unsolved.

The first problem is that the electrolyte decomposes at the graphitesurface during initial charging (the first reaction in which lithium isoccluded in the graphite). A lithium secondary cell is charged anddischarged through the occlusion and release of lithium between thepositive and negative electrodes. For instance, in initial charging, ifelectricity corresponding to 20 units out of 100 units of lithiumoccluded in the positive electrode is consumed by electrolytedecomposition, this means that only 80 units of lithium end up beingoccluded in the negative electrode. If there is no electrolytedecomposition, the maximum 100 units of lithium can be utilized incharging and discharging, but in the above example, only a maximum of 80units of lithium can be utilized, so the electrolyte decompositionreaction in initial charging contributes to lower cell capacity.

The second problem is that with natural graphite particles grown fromgraphite crystals, which is the to allow greater occlusion and releaseof lithium, or with artificial graphite particles produced by thegraphitization of coke, the interlayer graphite bonds are broken bypulverization, resulting in graphite particles with a higher aspectratio, which are referred to as flakes. These graphite flakes end upbeing oriented in the planar direction of a collector when kneaded witha binder and applied over the collector to produce an electrode. As aresult, repeated occlusion and release of lithium into and out of thegraphite particles causes the graphite layers to expand and contract,creating strain, which decreases adhesion between the oriented graphiteparticles and the collector, so cycling characteristics and quickcharging and discharging characteristics suffer.

In regard to the first problem, suppressing electrolyte decomposition bycovering the surface of the graphite with an amorphous carbon layer hasbeen disclosed in Japanese Patent No. 2,643,035. For the second problem,the use of clustered graphite particles so that flat graphite particleswill remain unoriented has been disclosed in Japanese Laid-Open PatentApplication No. H10-158005, while the use of flake-like natural graphitemodified particles having a cabbage-like appearance and a circularity ofat least 0.86 has been disclosed in Japanese Laid-Open PatentApplication No. H11-263612.

DISCLOSURE OF THE INVENTION

However, the technique disclosed in Japanese Patent No. 2,643,035, inwhich electrolyte decomposition is suppressed by covering the graphitesurface with an amorphous carbon layer, does not necessarily result in alithium secondary cell with large capacity. Graphite covered withamorphous carbon has a higher average charging and discharging voltagethan uncovered graphite, so the amorphous carbon leads to new problems,such as a reduction in the amount of lithium occluded and released underpractical usage conditions, and a reduction in the amount of negativeelectrode active material that can be packed because of a lower truespecific gravity.

Meanwhile, with the technique disclosed in Japanese Laid-Open PatentApplication No. H10-158005, involving the use of clustered graphiteparticles so that a plurality of flat graphite particles will remainunoriented, or the technique disclosed in Japanese Laid-Open PatentApplication No. H11-263612, involving the use of flake-like naturalgraphite modified particles having a cabbage-like appearance and acircularity of at least 0.86, the electrolyte decomposition reactioncannot be suppressed in initial charging, so these approaches do notyield a lithium secondary cell with large capacity.

In view of this, it is an object of the present invention to provideartificial graphite particles and a method for manufacturing the same,with which the electrolyte decomposition reaction in initial charging issuppressed and irreversible capacity is reduced, without sacrificing theadvantage of a graphite negative electrode of being capable of occludingand releasing a large amount of lithium, thereby increasing the capacityof a lithium secondary cell, and to provide a nonaqueous electrolytesecondary cell negative electrode in which these artificial graphiteparticles are used, and a method for manufacturing this electrode, aswell as a lithium secondary cell that makes use of this nonaqueouselectrolyte secondary cell negative electrode.

As a result of diligent research aimed at achieving the stated object,the inventors learned that in the electrolyte decomposition reactionthat occurs in the initial charging of a graphite negative electrode,solvent molecules attempt to penetrate (cointercalate) between thegraphite layers in a state of coordination with lithium ions, resultingin decomposition due to the large steric hindrance of the solventmolecules themselves (Journal of Power Sources, Vol. 54, p. 288 (1995)).

In view of this, in the present invention the inventors first improvedthe surface portion of the graphite where the lithium penetrates, sothat the above-mentioned electrolyte decomposition reaction could besuppressed. Decomposition of electrolyte is believed to be mostprevalent when the graphite crystallinity is high all the way up to nearthe surface, so the inventors attempted to render the surface ofgraphite particles amorphous. The electrolyte decomposition reaction wasdiminished by a method in which a carbonaceous material was mixed withgraphite and recalcined, and a method in which the surface was coatedwith amorphous carbon by chemical vapor deposition (CVD). Nevertheless,new problems arose from the use of amorphous carbon. For example, theaverage charging and discharging voltage ended up higher than that ofgraphite and the amount of occluded and released lithium decreased underpractical usage conditions, and the true specific gravity decreased,resulting in a reduction in the amount of negative electrode activematerial that could be packed. It was therefore found that the capacityof a lithium secondary cell could not be increased.

As a result of further investigation in light of the above, theinventors discovered that the object of the present invention could beachieved by using graphite particles in which the crystallinity isreduced in just the surface layer for the negative electrode material ofa lithium secondary cell, in order to suppress the electrolytedecomposition reaction without sacrificing the advantage of highercapacity provided by graphite. It was also found that such graphiteparticles could be manufactured by a specific manufacturing method.

Specifically, the artificial graphite particles of the present inventionare characterized by having a secondary particle structure in which aplurality of primary particles composed of graphite are clustered orbonded together, wherein the primary particles have a layer structurewith a polyhedral edge portion.

The method for manufacturing artificial graphite particles of thepresent invention is characterized in that raw material graphiteparticles having a layer structure with a polyhedral edge portion arepassed through a gap between two members positioned with this gaptherebetween, on or both of which is rotating.

The nonaqueous electrolyte secondary cell negative electrode of thepresent invention has graphite particles that occlude or release alkalimetal ions, affixed by an organic binder to a metal foil surface,wherein this nonaqueous electrolyte secondary cell negative electrode ischaracterized in that the above-mentioned graphite particles arecomposed of the above-mentioned artificial graphite particles of thepresent invention. The method of the present invention for manufacturinga nonaqueous electrolyte secondary cell negative electrode is a methodfor manufacturing a nonaqueous electrolyte secondary cell negativeelectrode having graphite particles that occlude or release alkali metalions, wherein this method is characterized in that the above-mentionedgraphite particles are manufactured by the above-mentioned method formanufacturing artificial graphite particles of the present invention.

The lithium secondary cell of the present invention has a laminate,produced by the successive lamination of a negative electrode capable ofoccluding and releasing lithium, a separator, and a positive electrodecapable of occluding and releasing lithium, and a nonaqueous electrolytein a container, characterized in that the above-mentioned negativeelectrodes comprise the negative electrode of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section illustrating an example of a cylindricallithium secondary cell of the present invention;

FIG. 2 is a cross section illustrating an example of a button-typelithium secondary cell of the present invention;

FIG. 3 is a diagram corresponding to a scanning electron micrograph(SEM) of the raw material graphite particles of the present invention;

FIG. 4 is a diagram corresponding to a scanning electron micrograph(SEM) of the artificial graphite particles of the present invention;

FIG. 5 is a schematic cross section of the artificial graphite particlesof the present invention;

FIG. 6 is a schematic cross section of the artificial graphite particlesof the present invention;

FIG. 7 is a diagram corresponding to a transmission electron micrograph(TEM) of the artificial graphite particles of the present invention;

FIG. 8 is a graph of the thermogravimetric change (TG) in the artificialgraphite particles of the present invention and conventional graphiteparticles;

FIG. 9 is a graph of the differential thermal amount change (DTA) in theartificial graphite particles of the present invention and conventionalgraphite particles;

FIG. 10 is a diagram of the electrochemical cell used in the presentinvention;

FIG. 11 shows a cyclic voltammogram produced in Example 2 andComparative Example 2;

FIG. 12 is a diagram corresponding to a scanning electron micrograph(SEM) of the lithium secondary cell negative electrode of the presentinvention in an overcharged state;

FIG. 13 is a diagram corresponding to a scanning electron micrograph(SEM) of a conventional lithium secondary cell negative electrode in anovercharged state;

FIG. 14 is a diagram corresponding to a scanning electron micrograph(SEM) of an unused lithium secondary cell negative electrode of thepresent invention; and

FIG. 15 is a diagram corresponding to a scanning electron micrograph(SEM) of an unused conventional lithium secondary cell negativeelectrode.

BEST MODES FOR CARRYING OUT THE INVENTION

Artificial Graphite Particles

The artificial graphite particles of the present invention have asecondary particle structure in which a plurality of primary particlescomposed of graphite are clustered or bonded together, wherein theprimary particles have a layer structure with a polyhedral edge portion.

It is known that in the penetration of lithium ions between graphitelayers, solvent molecules coordinated with the lithium ions work theirway in, resulting in the decomposition of the solvent, but when lithiumpenetrates the artificial graphite particles of the present inventionhaving the above-mentioned structure, even if solvent moleculescointercalate between the graphite layers in a state of coordinationwith lithium, because the graphite layer at the edge portion is bent ina polyhedral shape, the graphite layers spread out more readily than ingraphite with high crystallinity, and therefore there is less effect ofsteric hindrance and solvent decomposition is minimized. Specifically,employing a structure in which the graphite layer at the edge portion isbent in a polyhedral shape suppresses the decomposition reaction of thesolvent in the electrolyte and so forth in a nonaqueous electrolytesecondary cell. The above structure can be confirmed by transmissionelectron microscopy (TEM).

Therefore, when the artificial graphite particles of the presentinvention are used in the negative electrode of a nonaqueous electrolytesecondary cell negative electrode, there is less irreversible capacityin initial charging and discharging, the quick discharge loadcharacteristics are excellent, and the cycling life is longer, amongother advantages, and furthermore the charge receptability is improved.The reason for this is believed to be that in a lithium secondary cell,the lithium is deposited in dendrite form with a conventional negativeelectrode material during overcharging, whereas the deposition is in theform of particles or moss with the artificial graphite particles of thepresent invention, the effect of which is to enhance safety duringovercharging.

In the artificial graphite particles of the present invention, it ispreferable if the outermost surface of the secondary particles has asurface layer in a low-crystallinity or amorphous state. Because theedge portion of the primary particles is bent in a polyhedral shape withthe artificial graphite particles of the present invention, as mentionedabove, there are more crystal defects at the outermost surface, and thesurface of the secondary particles tends to be in a state of lowcrystallinity (more crystal defects) or in some places even amorphous.This structure can also be confirmed by transmission electron microscopy(TEM), and the low crystallinity or amorphousness of the surface can beexamined by Raman spectrum measurement.

The peak intensity ratio (R=I₁₃₆₀/I₁₅₈₀) between the peak (I₁₃₆₀) near1360 cm⁻¹ and the peak (I₁₅₈₀) near 1580 cm⁻¹ in Raman spectrummeasurement of the artificial graphite particles of the presentinvention is preferably 0.1≦R≦0.5, with 0.1≦R≦0.4 being more preferable,0.1≦R≦0.3 better yet, and 0.1≦R≦0.2 particularly favorable. The peaknear 1360 cm⁻¹ originates in the amorphous portion of the graphiteparticle surface, while the peak near 1580 cm⁻¹ seems to be attributableto the graphite crystal portion, and from such standpoints as theabove-mentioned effect of suppressing solvent decomposition, it ispreferable for the artificial graphite particles of the presentinvention to have a Raman peak intensity ratio attributable to theamorphous and graphite crystal portions to be with the range specifiedabove.

In the artificial graphite particles of the present invention, it ispreferable if the above-mentioned secondary particle structure is one inwhich a plurality of primary particles composed of graphite areclustered or bonded together in a non-parallel manner, the structurehaving voids inside the secondary particles.

The voids present inside the artificial graphite particles can bemeasured in terms of pore volume measured by mercury porosimetry, and itis preferable for the pore volume found by this method to be from 0.1 to0.5 cm³/g. The quick discharge characteristics will tend to be superiorif the pore volume is within this range. If the pore volume is less than0.1 cm³/g, not enough electrolyte will be retained for use in thenonaqueous electrolyte secondary cell, and the quick dischargecharacteristics will suffer, but if 0.5 cm³/g is exceeded, the binderthat is mixed with the artificial graphite particles to form thenonaqueous electrolyte secondary cell negative electrode will get intothe pores and reduce contact with the artificial graphite particles andthe collector, making it less likely that the nonaqueous electrolytesecondary cell will have good cycling characteristics.

It is also preferable if the artificial graphite particles of thepresent invention have a bulk density of at least 0.8 g/cm³. If the bulkdensity is at least this high, coating of electrodes for nonaqueouselectrolyte secondary cells will be excellent and adhesion with thecollector will be superior. Put another way, electrode coatability willtend to decrease if the bulk density is less than 0.8 g/cm³. “Bulkdensity” as used herein refers to the value obtained by putting graphiteparticles in a vessel, repeatedly tapping the vessel until there is nofurther change in the particle volume, and then taking a measurement.

It is preferable if the specific surface area of the artificial graphiteparticles of the present invention is 3 to 6 m²/g. The quick chargingand discharging characteristics of the nonaqueous electrolyte secondarycell will tend to be better and safety higher if the specific surfacearea is within the above range. Specifically, the quick charging anddischarging characteristics will tend to suffer if the specific surfacearea is less than 3 m²/g, but cell safety will tend to decrease if 6m²/g is exceeded.

It is also preferable if the surface oxygen concentration is 1.0 to 4.0atom % (and even more preferably, 1.0 to 3.0 atom %). The surface oxygenconcentration can be measured by X-ray photon spectrometry (XPS). If thesurface oxygen concentration is between 1.0 and 4.0 atom %, thestability of the electrode mix paste obtained by mixing the artificialgraphite particles with a binder and a solvent, the affinity with theelectrolyte in the nonaqueous electrolyte secondary cell, the adhesionwith the binder, and so forth will all tend to be better, which affordsbetter negative electrode characteristics in a nonaqueous electrolytesecondary cell.

If the surface oxygen concentration is less than 1.0 atom %, it will bedifficult to improve electrode coatability and electrode adhesion, butif the surface oxygen concentration is over 4.0 atom %, the quickcharging and discharging characteristics will tend to be inferior.Artificial graphite particles having a surface oxygen concentrationwithin the above range can be easily obtained by the method formanufacturing artificial graphite particles of the present invention,which is described in detail below. This manufacturing method involvessubjecting the raw material graphite particles to a grinding treatment(frictional pulverization treatment), and it is believed that a surfaceoxygen concentration within the above range is attained by the oxidationof the graphite particle surfaces by the surrounding oxygen and the heatgenerated by friction.

In the present invention, it is preferable if the artificial graphiteparticles meet all of the above conditions. Specifically, it ispreferable if the bulk density is at least 0.8 g/cm³, the specificsurface area is 3 to 6 m²/g, and the surface oxygen concentration asmeasured by X-ray photon spectrometry (XPS) is 1.0 to 4.0 atom %.

It is preferable with the artificial graphite particles of the presentinvention if weight reduction and heat generation occur at a temperatureof at least 640° C., and the weight reduction caused by heating for 30minutes at 650° C. is less than 3%, in thermogravimetric-differentialthermal analysis (TG-DTA) under an air flow.

Because the weight reduction during the above-mentioned measurement is5% or more with a conventional carbon material, the artificial graphiteparticles of the present invention undergo less weight reduction than aconventional carbon material. The carbon materials used in conventionallithium secondary cells have low graphite crystallinity and the crystalsinside the particles contain numerous defects or undeveloped amorphousportions, which seems to be why the exothermic commencement temperatureis lower and the weight reduction is greater than with the artificialgraphite particles of the present invention. On the other hand, with theartificial graphite particles of the present invention, thecrystallinity is high and only the outermost surface is bent in apolyhedral shape and rendered amorphous, which seems to be why theexothermic commencement temperature is higher and the weight reductionis less.

Artificial graphite particles that exhibit weight reduction such as thiscan be manufactured by the method for manufacturing artificial graphiteparticles of the present invention discussed below, and artificialgraphite particles obtained in this way exhibit weight reduction andexothermic behavior at 640° C. or higher. In contrast, with the carbonmaterials used in conventional lithium secondary cells, exothermicbehavior is noted at around 600° C., so the exothermic commencementtemperature of the artificial graphite particles of the presentinvention is higher than that of conventional carbon materials.

It is preferable with the artificial graphite particles of the presentinvention if the average particle size is 10 to 50 μm, the true densityis at least 2.2 g/cm³, and the spacing d002 of the (002) plane in thegraphite is less than 0.337 nm. If the average size of the artificialgraphite particles is greater than 50 μm, the electrode surface willtend to be bumpy, which can lead to short-circuiting when used in anonaqueous electrolyte secondary cell. On the other hand, if the averageparticle size is less than 10 μm, the specific surface area of thegraphite particles will be larger, so electrode coatability willdecrease, and microparticles will tend to lower the safety of the cell.If the true density is less than 2.2 g/cm³ and d002 is 0.337 nm orhigher, this means that the crystallinity of the graphite is low, andthere will be a reduction in the amount of lithium that can be occludedand released, so the charging and discharging capacity of the lithiumsecondary cell negative electrode will tend to decrease.

It is preferable if the viscosity of a paste (electrode mix paste)obtained by kneading the artificial graphite particles with thefollowing binder (a) and solvent (b) is 0.3 to 1.6 Pa·s (more preferably0.6 to 1.3 Pa·s, and even more preferably 1.0 to 1.3 Pa·s) at atemperature of 25° C. and a shear rate of 4 sec⁻¹.

-   -   (a) Polyvinylidene fluoride, where the weight ratio of the        binder to the artificial graphite particles is 1:9, and the        polyvinylidene fluoride is a polyvinylidene fluoride that        exhibits a solution viscosity of 550±100 mPa·s when 12.0±0.5 wt        % N-methyl-2-pyrrolidone solution is produced.    -   (b) N-methyl-2-pyrrolidone, in a weight of 45% with respect to        the total amount of the paste.

An example of the above-mentioned polyvinylidene fluoride (a) isPolyvinylidene Fluoride #1120 made by Kureha Chemical. The viscosity ofthe electrode mix paste in the present invention is measured by thefollowing method. An electrode mix paste is produced by mixing graphiteparticles and polyvinylidene fluoride (#1120 from Kureha Chemical) in aweight ratio of 90:10 and adding N-methyl-2-pyrrolidone so that thecombined solids concentration of the graphite particles and thepolyvinylidene fluoride will be 45 wt %, and the viscosity is measuredwith a Brookfield Model DV-III at 25° C. and a shear rate of 4 sec⁻¹.

When a nonaqueous electrolyte secondary cell negative electrode isproduced using artificial graphite particles in which the viscosity ofthe electrode mix paste is over 1.6 Pa·s, a large amount of solvent hasto be used to adjust the viscosity for coating, which raises the solventcost and requires more time and energy in the drying of the electrode,and electrode adhesion also decreases, among other problems. On theother hand, if the viscosity of the electrode mix paste is less than 0.3Pa·s, when the resulting artificial graphite particles are used toproduce a nonaqueous electrolyte secondary cell negative electrode andthen used in a nonaqueous electrolyte secondary cell, discharge loadcharacteristics tend to suffer.

It is preferable if a paste has a shear rate dependence (TI) of theviscosity at 25° C. of 2.0 to 4.0 (with 2.0 to 3.5 being even better,and 2.6 to 3.0 being especially good), the paste being obtained bykneading the artificial graphite particles with the following binder (a)and solvent (b) and the paste having a viscosity of 1.0 Pa·s at 25° C.and a shear rate of 4 sec⁻¹, where TI is defined by the formulaTI=(viscosity at shear rate of 4 sec⁻¹)/(viscosity at shear rate of 40sec¹).

-   -   (a) Polyvinylidene fluoride, where the weight ratio of the        binder to the artificial graphite particles is 1:9, and the        polyvinylidene fluoride is a polyvinylidene fluoride that        exhibits a solution viscosity of 550±100 mPa·s when 12.0±0.5 wt        % N-methyl-2-pyrrolidone solution is produced.    -   (b) N-methyl-2-pyrrolidone.

An example of the above-mentioned polyvinylidene fluoride (a) isPolyvinylidene Fluoride #1120 made by Kureha Chemical. The shear ratedependence (TI) in the present invention is measured by the followingmethod. An electrode mix paste is produced by mixing graphite particlesand polyvinylidene fluoride (#1120 from Kureha Chemical) in a weightratio of 90:10 and adding N-methyl-2-pyrrolidone to this mixture so thatthe viscosity will be 1.0 Pa·s at 25° C. and a shear rate of 4 sec⁻¹ asmeasured with a measured with a Brookfield Model DV-III. The viscosityof the electrode mix paste thus obtained is measured at 25° C. and ashear rate of 4 sec⁻¹ along with the viscosity at 25° C. and a shearrate of 40 sec⁻¹, and the shear rate dependence (TI) of the electrodemix paste viscosity is calculated from the above formula.

When a nonaqueous electrolyte secondary cell negative electrode isproduced using artificial graphite particles with which the shear ratedependence (TI) is over 4.0, there may be a decrease in the smoothnessof the produced negative electrode surface, and there may be separationin the coating surface at high speed, among other problems. On the otherhand, if a nonaqueous electrolyte secondary cell negative electrode isproduced using artificial graphite particles with which the shear ratedependence (TI) is less than 2.0, the discharge load characteristics ina cell tend to suffer.

Preferred embodiments of the artificial graphite particles of thepresent invention were described above, and it is preferable for theartificial graphite particles of the present invention to possess atleast one of the preferred characteristics discussed above, and it isparticularly favorable for it to possess all of these characteristics.

Method for Manufacturing Artificial Graphite Particles

The method for manufacturing artificial graphite particles of thepresent invention is characterized in that raw material graphiteparticles are passed through a gap between two members positioned withthe gap therebetween, one or both of which is rotating. Thismanufacturing method allows the artificial graphite particles of thepresent invention having the above-mentioned characteristics to bemanufactured with ease.

In the above-mentioned manufacturing method, it is preferable if the rawmaterial graphite particles are passed through the gap between twomembers disposed with their planes facing each other and with thedesired gap therebetween, with at least one of the members rotatingrelatively, so that at least the surfaces of the raw material graphiteparticles are subjected to a grinding treatment.

Furthermore, it is preferable if the raw material graphite particles aresupplied in between a fixed member and a rotating rotary member disposedat the lower part of the fixed member, these members being disposed withtheir planes facing each other and with the desired gap therebetween,through the feed port of the fixed member, provided at a locationcorresponding to the rotational center of the rotary member, and made topass through this gap, thereby subjecting at least the surfaces of theartificial graphite particles to a grinding treatment.

It is preferable if the two members rotate in opposite directions in theabove-mentioned manufacturing method. There are no particularrestrictions on the material of these two members, but a ceramicmaterial such as alumina, silicon carbide, or silicon nitride isfavorable in that there is less impurity contamination of the treatedgraphite particles.

A mortar type of grinding apparatus comprising two (upper and lower)grinders with which the gap between the two plates can be adjusted, forexample, can be used to implement the above manufacturing method. Withthis apparatus, for example, the raw material is centrifugally fed intothe gap between the upper and lower grinders, and the raw materialgraphite particles can be subjected to a grinding treatment (frictionalpulverization) by the compression, shear, rolling friction, and so forththus generated. Examples of commercially available apparatusesconstructed as above include the mortar pulverizer (Glow Mill) made byGlow Engineering, the Premax made by Chuo Kakoki Shoji, and theSerendipiter and Super Masscolloider made by Masuko Sangyo.

In the method for manufacturing artificial graphite particles of thepresent invention, it is preferable if the size of the gap between thetwo members is 0.5 to 20 times the average size of the raw materialgraphite particles. The average particle size can be measured with aparticle size distribution measurement apparatus that makes use of laserlight scattering (such as the SALD-3000 made by Shimadzu Seisakusho).The size of the gap between the members is controlled as the clearancebetween the upper and lower members (such as the plates of grinders orthe like), and can be set as desired, with zero being the point at whichthe upper and lower members lightly touch. If the gap between themembers is less than 0.5 times the average particle size, the particleswill be overly fine, which tends to make it difficult to manufacture theartificial graphite particles of the present invention having thecharacteristics described above. On the other hand, the effect of thegrinding treatment will tend to be diminished if the clearance betweenthe members is more than 20 times the average particle size.

Also, the speed of the two members as the graphite particles are passedthrough the gap between the members, one or both of which are rotating,along with the gap between the rotating members and the size (diameter)of the members, affects the rate at which the raw material graphiteparticles are ground. As this speed rises, so does the grinding rate.There are no particular restrictions on this speed in the presentinvention, but an outer peripheral speed (if the members are disks) of15 to 40 m/sec is favorable. The grinding rate and the manufacturingefficiency will both tend to decrease if the outer peripheral speed istoo low.

In the present invention, the grinding treatment in which the rawmaterial graphite particles are passed through a gap between two membersthat are positioned with a gap therebetween, one or both of which arerotating, can be performed in one or more passes. If the treatmentconsists of two or more passes, the bulk density can be increased morethan with a single pass. In this case, the grinding treatment conditions(such as the size of the gap between the two members (one or both ofwhich are rotating), the speed of the members, and the raw material feedrate) may be the same as or different from the conditions in theimmediately preceding treatment.

The manufacturing method of the present invention can be either a dry ora wet process. “Dry process” as used herein is one in which the rawmaterial graphite particles are passed through the gap between the twomembers (one or both of which are rotating), while “wet process” is onein which the raw material graphite particles are treated after beingdispersed in a suitable solvent. A wet process requires that theartificial graphite particles in the solvent be separated after thetreatment. Water or an organic solvent such as an alcohol can be used asthis solvent. Compared to a wet process, a dry process affords a higherbulk density, and is therefore preferable because the resultingartificial graphite particles have better electrode coatability andelectrode adhesion. Furthermore, a dry process entails no step ofdispersing the raw material graphite particles in a solvent prior to thegrinding treatment, and no step of separating the artificial graphiteparticles from the solvent after the grinding treatment, so theartificial graphite particles can be manufactured at lower cost.

In the manufacturing method of the present invention, it is preferableif the raw material graphite particles are massive artificial graphite.It is also preferable if the raw material graphite particles have asecondary particle structure in which a plurality of primary particlescomposed of graphite are clustered or bonded together, and the primaryparticles within the secondary particles have planes of orientation thatare not parallel to each other. In this case, it is preferable if theaspect ratio of the primary particles is 5 or less.

It is particularly favorable if the raw material graphite particles havea structure in which a plurality of flat graphite particles areclustered or bonded together in a non-parallel manner, the aspect ratiois 5 or less (preferably 1 to 3), and there are voids in the particles.The aspect ratio is found by measuring the minor and major diameters ofindividual particles in an SEM photograph of the graphite particles, andfinding the ratio of the major diameter to the minor diameter. Any 100particles are selected and the ratio found as above, and the averagethereof is determined as the aspect ratio.

Massive raw material graphite particles already having voids in theinterior of the particles are obtained by combining a graphitizableaggregate, a graphitization catalyst, and a binder that will bind thesetogether, then pre-calcining and graphitizing this mixture. Fluid coke,needle coke, and various other types of coke can be used as thegraphitizable aggregate. It is also possible to use natural orartificial graphite that has already been graphitized. The binder can bepetroleum, coal, artificial pitch, or tar, and a material that can begraphitized in the same manner as the aggregate is preferred. Thegraphitization catalyst can be a carbide, oxide, or nitride of silicon,iron, nickel, titanium, boron, or the like.

The pre-calcination and graphitization are preferably carried out in anatmosphere in which the aggregate and the binder will not readilyoxidize, such as a nitrogen atmosphere, an argon atmosphere, or avacuum. The pre-calcination should be performed at a temperature of 400to 1000° C., and the graphitization at a temperature of at least 2000°C. At the same time, the graphitization catalyst is eliminated at atemperature of 2000° C. or higher, and pores are formed in its place. Itis even better for the graphitization temperature to be 2500° C. orhigher, and 2800° C. or higher is best because it yields graphite withhigh crystallinity. If the graphitization temperature is under 2000° C.,the development of graphite crystals will be poor, and thegraphitization catalyst will remain behind in the graphite particles,which will tend to lower the charging and discharging capacity.

The amount in which the graphitization catalyst is added is preferable 1to 50 weight parts per combined 100 weight parts of graphitizableaggregate or graphite and graphitizable binder. If the amount is lessthan 1 weight part, development of artificial graphite particle crystalswill be poor and the charging and discharging capacity will tend to belower when the particles are used in a nonaqueous electrolyte secondarycell. On the other hand, if the amount is over 50 weight parts, uniformmixing will be difficult and the material will tend to be more difficultto work with.

Because the graphitized material obtained as above is in the form of ablock, it is preferably first pulverized. There are no particularrestrictions on the pulverization method, but a jet mill, vibratingmill, hammer mill, or the like can be used. The average particle sizeafter pulverization should be 100 μm or less, with a range of 10 to 50μm being preferable because coatability will be better. Also, thepulverized powder may be subjected to cold hydrostatic pressing ifneeded. The massive artificial graphite manufactured as above ispreferably used as the raw material graphite particles.

Nonaqueous Electrolyte Secondary Cell Negative Electrode, and Method forManufacturing this Electrode

The nonaqueous electrolyte secondary cell negative electrode of thepresent invention has graphite particles that occlude or release alkalimetal ions, affixed by an organic binder to a metal foil surface,wherein this nonaqueous electrolyte secondary cell negative electrode ischaracterized in that the above-mentioned graphite particles arecomposed of the artificial graphite particles of the present inventiondescribed above. In this case, it is preferable if the artificialgraphite particles of the present invention have been manufactured bythe above-mentioned method of the present invention for manufacturingartificial graphite particles.

When a negative electrode such as this is applied to a nonaqueouselectrolyte secondary cell, the electrolyte (such as a solution obtainedby dissolving LiPF₆ in an amount of 1 mol/dm³ in a mixed solvent ofethylene carbonate (EC) and ethyl methyl carbonate (EMC) with avolumetric ratio of 1:1) is less prone to decomposition. Also, thedischarge capacity of the nonaqueous electrolyte secondary cell ishigher and the irreversible capacity can be reduced.

The present invention provides a method for manufacturing a nonaqueouselectrolyte secondary cell negative electrode having graphite particlesthat occlude or release alkali metal ions, wherein this method formanufacturing a nonaqueous electrolyte secondary cell negative electrodeis characterized in that the above-mentioned graphite particles aremanufactured by the above-mentioned method for manufacturing artificialgraphite particles of the present invention. This manufacturing methodallows a nonaqueous electrolyte secondary cell negative electrodeexhibiting the above-mentioned characteristics to be obtained with ease.

Lithium Secondary Cell

The lithium secondary cell of the present invention has a laminate,produced by the successive lamination of a negative electrode capable ofoccluding and releasing lithium, a separator, and a positive electrodecapable of occluding and releasing lithium, and a nonaqueous electrolytein a container, wherein this lithium secondary cell is characterized inthat the above-mentioned negative electrode is composed of theabove-mentioned nonaqueous electrolyte secondary cell negative electrodeof the present invention. In this case, it is preferable if the lithiumoccluded on the surface of the graphite particles is deposited in theform of particles or moss (this refers to a state in which the depositedlithium metal covers the negative electrode material particlessubstantially uniformly).

When the lithium is deposited in the form of particles or moss, there isvery little irreversible capacity in initial charging and discharging,the quick discharge load characteristics are excellent, and the chargingand discharging cycling life is long, among other such advantages, andfurthermore charge receptability tends to be improved.

FIG. 1 is a cross section illustrating an example of a lithium secondarycell in which the artificial graphite particles of the present inventionare used in the negative electrode. A positive electrode 10, a separator11, and a negative electrode 12 are housed in a cell can 13 in a stateof being coiled such that the positive electrode 10, the separator 11,and the negative electrode 12 are laminated in that order. A positiveelectrode tab 14 is attached to the positive electrode 10, a negativeelectrode tab 15 is attached to the negative electrode 12, the positiveelectrode tab 14 is connected to a cell inner lid 16, and the negativeelectrode tab 15 is connected to the cell can 13. A safety valve(current shut-off valve) 17 is connected to the cell inner lid 16. Ifthe internal pressure rises over 10 atmospheres, the safety valve(current shut-off valve) 17 is deformed until it is electricallyisolated from the cell inner lid 16. A cell outer lid 20, the safetyvalve (current shut-off valve) 17, an insulating plate 19, and the cellinner lid 16 are laminated at their ends in this order and held in placeby a gasket 18.

FIG. 2 is a cross sectional front view illustrating an example of abutton-type lithium secondary cell in another example of the presentinvention. A pellet-shaped positive electrode 21 and a pellet-shapednegative electrode 22 are laminated with a separator 23 in between, thepositive and negative electrodes touch a positive electrode can 24 and anegative electrode can 25, respectively, to effect electricalconduction, and the positive electrode can and the negative electrodecan are sealed with a gasket 26.

The negative electrode used in the lithium secondary cell of the presentinvention is obtained by adding an organic binder to the artificialgraphite particles of the present invention and kneading and moldingthis mixture into the form of a sheet, pellet, or the like. Examples oforganic binders include polyethylene, polypropylene, ethylene propylenepolymer, butadiene rubber, styrene butadiene rubber, and butyl rubber.Polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin,polyphosphazene, polyacrylonitrile, and other such macromolecularcompounds capable of conducting lithium ions are also suitable as thisorganic binder. The organic binder is preferably contained in an amountof 1 to 20 weight parts per 100 weight parts of the mixture ofartificial graphite particles and organic binder.

A sheet-form negative electrode can be manufactured by adding water oran organic solvent to the mixture of artificial graphite particles andorganic binder to create a paste, adjusting the paste viscosity, thencoating a collector with this paste and drying to remove the solvent,then press molding with a roll press or the like.

A foil, mesh, or the like of copper, nickel, stainless steel, or thelike can be used as the collector. The pellet-shaped negative electrodecan be manufactured by press molding the mixture of artificial graphiteparticles and organic binder in a metal mold.

Meanwhile, there are no particular restrictions on the active materialused for the positive electrode in the lithium secondary cell of thepresent invention, but compounds expressed by the chemical formulasLiM_(x)Co_(1−x)O₂, Li_(1+x)Mn_(2−x)O₄, and Li_(1+x)M_(y)Mn_(2−x−y)O₄ (Mis one or more of Fe, Ni, Cr, Mn, Al, B, Si, and Ti, x≧0, and y≧0) canbe used to particular advantage. A sheet- or pellet-form positiveelectrode can be manufactured in the same manner as the negativeelectrode using the above-mentioned active material. A foil or mesh ofaluminum, however, is used for the collector.

The solvent of the organic electrolyte used in the lithium secondarycell of the present invention is a mixed solvent obtained by adding oneor more members of the group consisting of dimethyl carbonate, diethylcarbonate, ethyl methyl carbonate, γ-butyrolactone, sulfolane, methylacetate, ethyl acetate, methyl propionate, ethyl propionate,dimethoxyethane, and 2-methyltetrahydrofuran to ethylene carbonate, andit is preferable for the volumetric percentage of the ethylene carbonateto be at least 0.1 and no more than 0.6. Meanwhile, one or more membersof the group consisting of LiPF₆, LiBF₄, LiAsF₆, LiClO₄, (C₂F₅SO₃)₂NLi,and (CF₃SO₃)₂NLi are used as the lithium salt in the present invention,the concentration of which should be between 0.5 and 1.5 mol/dm³.

The separator used in the lithium secondary cell of the presentinvention can be nonwoven cloth, woven cloth, microporous film, or acombination of these, whose main component is a polyolefin such aspolyethylene or polypropylene. Of these, the use of a microporous filmmade of polyethylene and having a thickness of 20 to 50 μm ispreferable.

EXAMPLES

Preferred examples of the present invention will now be described infurther detail, but the present invention is not limited to or by theseexamples.

Manufacture of Raw Material Graphite Particles and Measurement ofProperties Example 1a

100 weight parts of coke powder with an average particle size of 5 μm,30 weight parts tar pitch, 30 weight parts of silicon carbide with anaverage particle size of 48 μm, and 20 weight parts coal tar were mixedfor 1 hour at 270° C. The resulting mixture was pulverized, press moldedinto a pellet, and pre-calcined at 900° C. in nitrogen, after which thisproduct was graphitized at 2800° C. in an Acheson furnace. The graphiteblock obtained above was pulverized in a hammer mill and passed througha 200-mesh standard sieve to produce massive raw material graphiteparticles.

Next, using a Masscolloider (MK10-15J) made by Masuko Sangyo andequipped with an MKGC10-120 grinder, and with the grinder gap(clearance) set to 60 μm (with zero being the point at which the upperand lower member slightly touch), the above-mentioned raw materialgraphite particles were subjected to a grinding treatment by beingpassed through this gap, which yielded the artificial graphite particlesof the present invention. With the upper grinder stationary, the lowergrinder was rotated at 1500 rpm, and the raw material graphite particleswere fed through a feed port provided to the upper grinder in theportion corresponding to the center of the lower grinder. The artificialgraphite particles went through two passes of this treatment, and theupper and lower grinders have fine bumps on the opposing surfaces.

The raw material graphite particles and the artificial graphiteparticles of the present invention were examined for the followingproperties (1) to (10). Both types of particles were also examined byelectron microscope (SEM) as discussed below.

(1) Average particle size: measured with a Shimadzu Seisakusho laserdiffraction particle size distribution measurement apparatus(SALD-3000).

(2) Aspect ratio: the ratio of major diameter to minor diameter wasfound for 100 randomly selected graphite particles by scanning electronmicroscopic (SEM) observation, and the average thereof was used as arepresentative value.

(3) True density: Measured by the butanol method set forth in JIS R7212.

(4) Bulk density: A sample was put in a 200 mL glass graduated cylinderand tapped until there was no further change, at which point the samplevolume was measured, and this was divided by the sample weight.

(5) Graphite interlayer spacing (hereinafter referred to as spacingd002): The (002) spacing d002 of the graphite was measured using aPhilips X-ray diffractometer PW1730 (goniometer PW1050), with the Cu-Kαline rendered monochromatic with a Ni filter, and with high-puritysilicon used as an internal standard sample.

(6) Specific surface area: This was calculated by BET method after usinga Micromeritics ASAP2010 to measure the nitrogen absorption at thetemperature of liquid temperature by multi-point method.

(7) Pore volume: Pores within a range of 10 to 10⁵ nm were measured bymercury porosimetry using an Autoscan 33 made by Yuasa Ionics.

(8) Raman spectrum intensity ratio (hereinafter also referred to as“peak intensity ratio (R=I₁₃₆₀/I₁₅₈₀)”): A sample was mixed with KBrpowder and molded into tablets, and a Renishaw Raman scope (excitationwavelength: 532 nm, laser power: 60 mW, laser irradiation system:macrosample chamber 135° irradiation unit, light splitter: singlepolychrometer, detector: CCD, incident slit width: 100 μm, measurementtime: exposure time 15 minutes) was used to find the peak intensityratio (R=I₁₃₆₀/I₁₅₈₀) between the peak (I₁₃₆₀) near 1360 cm⁻¹ and thepeak (I₁₅₈₀) near 1580 cm⁻¹.

(9) Surface oxygen concentration: Measurement was made with an AXIS-165made by Shimadzu Seisakusho Kratos (monochromatic Al-Kα, 30 to 150 W (15kV, 2 to 10 mA), measured area: 0.3×0.7 mm², quantitative analysis PE:10 eV, qualitative analysis PE: 160 eV, detector uptake angle: 90°,sample: electrode prior to charging and discharging test), the peaksensitivity coefficient of each measured element was divided by the peaksurface area of each element and these quotients were summed, and theratio of the quotient obtained by dividing the peak sensitivitycoefficient of oxygen by the peak surface area of oxygen with respect tothe above-mentioned sum was calculated.

(10) Electrode mix paste viscosity: An electrode mix paste was producedby mixing graphite particles and polyvinylidene fluoride (#1120 made byKureha) in a weight ratio of 90:10 and adding N-methyl-2-pyrrolidone sothat the combined solids concentration of the graphite particles and thepolyvinylidene fluoride would be 45 wt %, and the viscosity of theelectrode mix paste was measured with a Brookfield Model DV-III at 25°C. and a shear rate of 4 sec⁻¹.

Table 1 shows the various property values of the raw material graphiteparticles and the artificial graphite particles of the presentinvention. It can be seen that the bulk density increased, the peakintensity ratio (R=I₁₃₆₀/I₁₅₈₀) increased, the surface oxygenconcentration increased, and the electrode mix paste viscosity decreasedin the artificial graphite particles of the present invention comparedto those of the raw material graphite particles. Furthermore, theelectrode mix paste viscosity was measured using electrode coatabilityand electrode adhesion as indices, and with the artificial graphiteparticles of the present invention, the paste viscosity was lower, theelectrode coatability was better, and the electrode adhesion wasimproved as compared to the raw material graphite particles.

TABLE 1 Example 1a Raw material graphite Graphite particles particlesAverage particle size (μm) 22.5 19.7 Aspect ratio 2.5 1.8 True density(g/cm³) 2.242 2.242 Bulk density (g/cm³) 0.71 0.93 Spacing d002 (nm)0.335 0.335 Specific surface area (m²/g) 3.6 3.7 Pore volume (cm³/g)0.87 0.5 Peak intensity ratio (R = 0.09 0.25 I₁₃₆₀/I₁₅₈₀) Surface oxygenconcentration 0.5 1.9 (atom %) Electrode mix paste viscosity 2.40 0.56(Pa · s)

Example 1d

100 weight parts of coke powder with an average particle size of 5 μm,40 weight parts tar pitch, 25 weight parts of silicon carbide with anaverage particle size of 48 μm, and 20 weight parts coal tar were mixedfor 1 hour at 270° C. The resulting mixture was pulverized, press moldedinto a pellet, and pre-calcined at 900° C. in nitrogen, after which thisproduct was graphitized at 3000° C. in an Acheson furnace. The graphiteblock obtained above was pulverized in a hammer mill and passed througha 200-mesh standard sieve to produce massive raw material graphiteparticles.

Next, using a Masscolloider (MK10-15J) made by Masuko Sangyo andequipped with an MKGC10-120 grinder, and with the grinder gap(clearance) set to 40 μm (with zero being the point at which the upperand lower members lightly touch), the above-mentioned raw materialgraphite particles were subjected to a grinding treatment by beingpassed through this gap, which yielded the artificial graphite particlesof the present invention. With the upper grinder stationary, the lowergrinder was rotated at 1500 rpm, and the graphite particles went throughone pass of this treatment.

Examples 1e to 1g

The artificial graphite particles of the present invention were obtainedin the same manner as in Example 1d, except that the grinder gap waschanged to 80 μm, 200 μm, and 300 μm in the various examples.

The properties (1) to (10) of the artificial graphite particles of thepresent invention obtained in Examples 1d to 1 g were examined in thesame manner as in Example 1a. Table 2 shows the various property valuesof the artificial graphite particles of the present invention inExamples 1d to 1 g.

TABLE 2 Example Example Example Example 1d 1e 1f 1g Grinder gap setting(μm) 40 80 200 300 Grinder gap size versus 1.9 3.7 9.3 14 averageparticle size of raw material graphite particles (times) Averageparticle size 19.8 20.0 21.0 21.2 (μm) Aspect ratio 1.7 1.8 1.9 2.0 Truedensity (g/cm³) 2.241 2.241 2.242 2.242 Bulk density (g/cm³) 0.94 0.900.86 0.83 Spacing d002 (nm) 0.335 0.335 0.335 0.335 Specific surfacearea 3.8 3.7 3.7 3.6 (m²/g) Pore volume (cm³/g) 0.18 0.22 0.3 0.32 Peakintensity ratio (R = 0.29 0.20 0.15 0.10 I₁₃₆₀/I₁₅₈₀) Surface oxygen 2.42.0 1.2 1.1 concentration (atom %) Electrode mix paste 0.50 0.60 0.800.80 viscosity (Pa · s)

It can be seen from Table 2 that with all of the artificial graphiteparticles of the present invention, the average particle size wasbetween 10 and 50 μm, the true density was at least 2.2 g/cm³, thespacing d002 of the (002) plane of the graphite was less than 0.337, thebulk density was at least 0.8 g/cm³, the specific surface area wasbetween 3 and 6 m²/g, the pore volume as measured by mercury porosimetrywas between 0.1 and 0.5 cm³/g, the peak intensity ratio (R=I₁₃₆₀/I₁₅₈₀)between the peak (I₁₃₆₀) appearing at 1360 cm⁻¹ and the peak (I₁₅₈₀)appearing at 1580 cm⁻¹ measured by Raman spectroscopy was at least 0.1and no more than 0.3 (0.1≦R≦0.3), and the surface oxygen concentrationas measured by X-ray photon spectrometry (XPS) was between 1 and 3 atom%. Also, with the artificial graphite particles of the presentinvention, the electrode mix paste viscosity was lower and the electrodecoatability and adhesion were both improved as compared to the rawmaterial graphite particles shown in Table 1. Further, it can be seenthat the electrode mix paste viscosity is particularly low (which isdesirable) because the size of the grinder gap versus the averageparticle size of the raw material graphite particles was between 0.5 and20 times in Examples 1d, 1e, 1f, and 1 g in Table 2.

Comparative Example 1a

The properties (1) to (10) of an artificial graphite powder with anaverage particle size of 25 μm were examined in the same manner as inExample 1a.

Comparative Example 1b

20 weight parts coal tar was added to 100 weight parts of a naturalgraphite powder with an average particle size of 25 μm and mixed for 1hour at 270° C., after which the resulting mixture was pulverized. Thismixed powder was press molded into a pellet and calcined at 900° C. innitrogen to produce graphite particles comprising natural graphitecovered by amorphous carbon. The properties (1) to (10) of the graphiteparticles thus obtained were examined in the same manner as in Example1a.

Comparative Example 1c

The artificial graphite particles produced in Example 1a were treated ina ball mill, and the properties (1) to (10) of the graphite particlesthus obtained were examined in the same manner as in Example 1a.

Comparative Example 1d

The artificial graphite particles produced in Example 1a were treated ina jet mill, and the properties (1) to (10) of the graphite particlesthus obtained were examined in the same manner as in Example 1a. Table 3shows the property values for the graphite particles in ComparativeExamples 1a to 1d.

TABLE 3 CE 1a CE 1b CE 1c CE 1d Average particle size (μm) 25.0 30.014.6 12.7 Aspect ratio 2.8 2.0 2.4 2.4 True density (g/cm³) 2.242 2.1852.242 2.242 Bulk density (g/cm³) 0.50 0.60 0.70 0.50 Spacing d002 (nm)0.335 0.335 0.335 0.335 Specific surface area (m²/g) 6.2 2 4.2 5.6 Porevolume (cm³/g) <0.1 <0.1 <0.1 <0.1 Peak intensity ratio (R = 0 0.50 0.320.45 I₁₃₆₀/I₁₅₈₀) Surface oxygen concentration 0 0 0.09 0.09 (atom %)Electrode mix paste viscosity 2.00 1.50 1.80 1.90 (Pa · s) [CE:Comparative Example]

It can be seen from Table 3 that with the graphite particles in thecomparative examples, the peak intensity ratio (R=I₁₃₆₀/I₁₅₈₀) of theRaman spectrum was less than 0.1 or greater than 0.3. It can also beseen that the surface oxygen concentration was less than 1 atom % andthe bulk density was less than 0.8 g/cm³. A comparison of Tables 1, 2,and 3 reveals that the artificial graphite particles of the presentinvention had a lower electrode mix paste viscosity than in thecomparative examples, so the electrode coatability was superior.

Examples 1h to 1k and Comparative Example 1e

Massive raw material graphite particles were produced in the same manneras in Example 1d. SEM photographs revealed that the raw materialgraphite particles thus obtained had a structure in which flat particleswere clustered or bonded together with a plurality of orientation planesin a non-parallel manner. These raw material graphite particles weresubjected to a grinding treatment using a Masscolloider (MK10-20J) madeby Masuko Sangyo and equipped with a GA10-120 grinder, with the upperand lower grinder gap (clearance) set to 40 μm (Example 1h), 80 μm(Example 1i), 200 μm (Example 1j), and 300 μm (Example 1k) from thepoint where the upper and lower grinders lightly touched. The rotationalspeed of the lower grinder of the Masscolloider (MK10-20J) was 1500 rpm,and the graphite particles were passed through one time. The properties(1) to (10) of the raw material graphite particles in Example 1h(Comparative Example 1e) and of the artificial graphite particles of thepresent invention obtained in Examples 1h to 1k were examined in thesame manner as in Example 1a. These property values are given in Table4.

TABLE 4 Example Example Example Example 1h 1i 1j 1k CE 1e Grinder gapsetting (μm) 40 80 200 300 N/A Average particle size 19.8 20.0 21.0 21.221.4 (μm) Aspect ratio 1.7 1.8 1.9 2.0 2.3 True density (g/cm³) 2.2412.241 2.242 2.242 2.242 Bulk density (g/cm³) 0.94 0.90 0.86 0.83 0.73Spacing d002 (nm) 0.335 0.335 0.335 0.335 0.335 Specific surface area3.8 3.7 3.7 3.6 3.5 (m²/g) Pore volume (cm³/g) 0.18 0.22 0.30 0.32 0.85Peak intensity ratio (R = I₁₃₆₀/ 0.20 0.16 0.13 0.10 0.09 I₁₅₈₀) Surfaceoxygen 2.4 2.0 1.2 1.1 0.9 concentration (atom %) Electrode mix paste0.50 0.60 0.80 0.95 2.50 viscosity (Pa · s) [CE: Comparative Example]

Examples 1o to 1t

Massive raw material graphite particles were produced in the same manneras in Example 1d. SEM photographs revealed that the raw materialgraphite particles thus obtained had a structure in which flat particleswere clustered or bonded together with a plurality of orientation planesin a non-parallel manner. These raw material graphite particles weresubjected to a grinding treatment using a Masscolloider (MKZA10-15J)made by Masuko Sangyo and equipped with an MKGC10-120 grinder. Here, therotational speed of the lower grinder was 1500 rpm, and the upper andlower grinder gap (clearance) set to 40 μm (Examples 1o and 1p), 60 μm(Examples 1q and 1r), and 80 μm (Examples 1s and 1t) from the pointwhere the upper and lower grinders lightly touched. In Examples 1p, 1r,and 1t, the graphite particles that had undergone a first grindingtreatment were put back into the apparatus and subjected to one moregrinding treatment.

The properties (1) to (10) of the artificial graphite particles of thepresent invention obtained in Examples 1o to It were examined in thesame manner as in Example 1a. In addition, the shear rate dependence(TI) of the electrode mix paste viscosity was measured by the followingmethod (11). These property values are compiled in Table 5.

(11) Shear rate dependence of electrode mix paste viscosity (hereinafterreferred to as “TI”): An electrode mix paste was produced by mixinggraphite particles and polyvinylidene fluoride (#1120 made by Kureha) ina weight ratio of 90:10 and adding N-methyl-2-pyrrolidone so that theviscosity of the electrode mix paste would be 1.0 Pa·s as measured witha Brookfield Model DV-III at 25° C. and a shear rate of 4 sec⁻¹. Theviscosity of the resulting electrode mix paste at 25° C. and a shearrate of 4 sec⁻¹ and its viscosity at 25° C. and a shear rate of 40 sec⁻¹were measured, the viscosity at 25° C. and a shear rate of 4 sec⁻¹ wasdivided by the viscosity at 25° C. and a shear rate of 40 sec⁻¹, andthis quotient was termed the shear rate dependence of the electrode mixpaste (TI).

TABLE 5 Ex. Ex. Ex. Ex. Ex. Ex. 1o 1p 1q 1r 1s 1t Grinder gap 40 40 6060 80 80 setting (μm) Number of 1 2 1 2 1 2 grinding treatments (times)Average 19.5 18.9 19.8 19.3 20.0 19.8 particle size (μm) Aspect ratio1.7 1.6 1.7 1.7 1.8 1.7 True density 2.241 2.241 2.241 2.241 2.241 2.241(g/cm³) Bulk density 0.877 0.905 0.860 0.890 0.846 0.881 (g/cm³) Spacingd002 0.335 0.335 0.335 0.335 0.335 0.335 (nm) Specific 3.7 3.9 3.0 3.42.9 3.3 surface area (m²/g) Pore volume 0.18 0.15 0.20 0.18 0.22 0.20(cm³/g) peak intensity 0.20 0.22 0.17 0.19 0.15 0.16 ratio (R = I₁₃₆₀/I₁₅₈₀) Surface oxygen 2.4 2.6 2.2 2.4 2.0 2.1 concentration (atom %)Electrode mix 1.28 0.98 1.18 1.00 1.58 1.10 paste viscosity (Pa · s) TIof electrode 3.0 2.6 3.0 2.7 3.2 2.8 mix paste viscosity

Comparative Examples 1f and 1 g

The raw material graphite particles in Example 1o were termedComparative Example 1f. In Comparative Example 1g, the raw materialgraphite particles of Comparative Example 1f were packed into a rubbermold, pressed at a pressure of 1.5 t/cm² in a cold hydrostatic press,and the molded article thus obtained was crushed in a pin mill andpassed through a 200-mesh sieve. The properties of these comparativeexamples were measured the same as in Examples 1o to 1t, the results ofwhich are given in Table 6.

TABLE 6 Comparative Comparative Example 1f Example 1g Average particlesize (μm) 20.3 20.1 Aspect ratio 1.8 1.7 True density (g/cm³) 2.2422.241 Bulk density (g/cm³) 0.650 0.750 Spacing d002 (nm) 0.335 0.335Specific surface area (m²/g) 2.7 3.9 Pore volume (cm³/g) 0.86 0.53 Peakintensity ratio (R = 0.11 0.11 I₁₃₆₀/I₁₅₈₀) Surface oxygen concentration0.9 1.0 (atom %) Electrode mix paste viscosity 2.78 1.9 (Pa · s) TI ofelectrode mix paste 3.9 3.7 viscosityStructure of Raw Material Graphite Particles and Graphite Particles

The raw material graphite particles and the artificial graphiteparticles of the present invention obtained in Example 1a were examinedby scanning electron micrograph (SEM). The raw material graphiteparticles are shown in FIG. 3, while the artificial graphite particlesof the present invention are shown in FIG. 4. As seen in FIG. 4, thestructure (secondary particle structure) is such that the primaryparticles within the secondary particles that made up the artificialgraphite particles of the present invention were clustered or bondedtogether so that the orientation planes were in a non-parallel manner.FIG. 5 shows the state of the secondary particles; in this secondaryparticle structure, primary particles composed of graphite 1 areclustered or bonded together, the planes of orientation of these primaryparticles are in a non-parallel manner with each other, and thesecondary particles include voids 2 surrounded by primary particles.

Further, the edge portions of the primary particles in the artificialgraphite particles of the present invention were examined bytransmission electron micrograph (TEM). As seen in the diagram of FIG. 6and the TEM of FIG. 7, the edge portion has a layer structure in whichthe graphite layer is bent in a polyhedral shape. When lithiuminfiltrates into the artificial graphite particles of the presentinvention, even if the solvent molecules should cointercalate betweenthe graphite layers, because the edge portions have this structure inwhich the graphite layers are bent in a polyhedral shape, the graphitelayers spread out more readily than in graphite with high crystallinity,and therefore there is less effect of steric hindrance and solventdecomposition is minimized.

Thermal Analysis of Graphite Particles

A TG/DTA6200 made by Seiko Denshi Kogyo was used to performthermogravimetric-differential thermal analysis (TG-DTA) under an airflow for the artificial graphite particles of the present inventionobtained in Example 1a (61 in FIG. 8, 71 in FIG. 9), the artificialgraphite particles of Comparative Example 1a (62 in FIG. 8, 72 in FIG.9), and the graphite particles comprising natural graphite covered withamorphous carbon of Comparative Example 2 (63 in FIG. 8, 73 in FIG. 9).The measurement conditions comprised an air (the atmosphere gas) flow of200 cm³/min, and heating from room temperature to 900° C. at atemperature elevation rate of 5° C./min. FIG. 8 shows thethermogravimetric change (TG), while FIG. 9 shows the differentialthermal amount change (DTA). It can be seen that the graphite particlesof Example 1a of the present invention had a higher exothermiccommencement temperature and temperature at which calcination causedweight reduction (a temperature of about 640° C. or higher) than did thegraphite particles in Comparative Examples 1a and 1b. Next, the weightreduction was examined when heating from room temperature to 650° C. wasperformed at a temperature elevation rate of 5° C./min and thetemperature was held for 30 minutes at 650° C.

The weight reduction in the graphite particles of Example 1a of thepresent invention, the artificial graphite particles of ComparativeExample 1a, and the graphite particles comprising natural graphitecovered with amorphous carbon of Comparative Example 1b was 2.8%, 5.1%,and 18.5%, respectively. It is believed that since the carbon materialsof Comparative Examples 1a and 1b have low graphite crystallinity andthe crystals inside the particles contain numerous defects orundeveloped amorphous portions, the exothermic commencement temperatureis lower and the weight reduction is greater than with the artificialgraphite particles of the present invention. On the other hand, it wasfound that with the artificial graphite particles of the presentinvention, the crystallinity is high and only the outermost surface isbent in a polyhedral shape and rendered amorphous. The artificialgraphite particles of the present invention produced in Examples 1d to 1g and the carbon materials of Comparative Examples 1c and 1d weresubjected to TG-DTA by the same method as described above, and as aresult it was found that, as shown in FIG. 7, the weight reduction wasless than 3% when the artificial graphite particles of the presentinvention were held at 650° C. for 30 minutes, and was 5% or higher withthe carbon materials of the comparative examples.

TABLE 7 Ex. Ex. Ex. Ex. Ex. CE CE CE CE 1a 1d 1e 1f 1g 1a 1b 1c 1dWeight 2.8 2.8 2.6 2.4 2.3 5.1 18.5 6.3 7.1 reduction (%) [CE:Comparative Example]

Electrolyte Decomposition Reaction in Nonaqueous Electrolyte SecondaryCell Negative Electrode Featuring Graphite Particles Example 2

A negative electrode was produced by the following method using theartificial graphite particles of the present invention obtained inExample 1a. Polyvinylidene fluoride (PVDF) was added in an amount of 10wt % as a binder to 90 wt % the artificial graphite particles of thepresent invention, and a suitable amount of N-methyl-2-pyrrolidone (NMP)was added as a solvent to create a paste. A collector (a copper foil)was coated with this paste in an amount of 11 mg/cm² per unit of surfacearea, after which the NMP was removed by drying. Press molding was thenperformed so that the density of the negative electrode mix would be 1.5g/cm³, and this product was used as a negative electrode.

The decomposition reaction of the electrode was examined by cyclicvoltammetry for the negative electrode of the present invention obtainedabove, using an electrochemical cell in which metallic lithium was usedfor the counter electrode and the reference electrode. FIG. 10 shows amodel cell used to evaluate the electrochemical characteristics of thenegative electrode in the present invention. In FIG. 10, 81 is anegative electrode, 82 is a metallic lithium counter electrode, 83 is ametallic lithium reference electrode, 84 is electrolyte, and 85 is aglass container. Evaluation was performed using a three-poleelectrochemical cell in which metallic lithium served as the standardpotential. The electrolyte 84 comprised a solution obtained bydissolving LiPF₆ in an amount of 1 mol/dm³ in a mixed solvent ofethylene carbonate (EC) and ethyl methyl carbonate (EMC) with avolumetric ratio of 1:1.

Comparative Example 2

The decomposition reaction of the electrode was examined by cyclicvoltammetry in the same manner as in Example 2, using the artificialgraphite particles with an average particle size of 25 μm of ComparativeExample 1a.

FIG. 11 is a shows a cyclic voltammogram illustrating the electrolytedecomposition reaction. In FIG. 11, 91 is the result of Example 2 and 92is the result of Comparative Example 5. A comparison of the reductioncurrent resulting from electrolyte decomposition at about 0.8 V shows itto be smaller with Example 2 than with Comparative Example 2, andtherefore there is less electrolyte decomposition reaction with theartificial graphite particles of the present invention.

Evaluation of Nonaqueous Electrolyte Secondary Cell Negative ElectrodeIn Which Graphite Particles Are Used Example 3a

A negative electrode was produced in the same manner as in Example 2,and the charging and discharging characteristics of the artificialgraphite particles of the present invention produced in Examples 1a to 1g were evaluated using the same electrochemical cell as that used inExample 2. “Charging” as used here refers to the occlusion of lithiuminto the negative electrode, while “discharging” refers to the releaseof lithium from the negative electrode. In the charging of the negativeelectrode, first constant current charging was performed at 0.5 mA/cm²,then constant voltage charging was performed at 5 mV from the point whenthe negative electrode potential reached 5 mV, and the charging wasconcluded at the point when the charging current attenuated to 0.02mA/cm². In the discharging of the negative electrode, constant currentcharging was performed at 0.5 mA/cm², and the discharging was concludedat the point when the negative electrode potential reached 1.5 V. Theirreversible capacity was found by subtracting the initial dischargecapacity from the initial charge capacity.

Comparative Example 3a

The charging and discharging characteristics of the conventionalgraphite particles used in Comparative Examples 1a to 1d were evaluatedin the same manner as in Example 3.

Table 8 shows the results of evaluating the charging and dischargingcharacteristics pertaining to irreversible capacity and initialdischarge capacity when the artificial graphite particles of the presentinvention were used in Example 3a, and the results of evaluating thecharging and discharging characteristics pertaining to irreversiblecapacity and initial discharge capacity when the conventional graphiteparticles in Comparative Example 3a were used.

The graphite particles of the present invention all had smallerirreversible capacity than the artificial graphite of ComparativeExample 1a and the graphite particles produced in Comparative Examples1c and 1d, and had larger discharge capacity and superior negativeelectrode characteristics compared to Comparative Example 1b. Further,Examples 1a, 1d, 1e, 1f, and 1 g, in which the size of the grinder gapwas between 0.5 and 20 times the average size of the raw materialgraphite particles used in the manufacture of the artificial graphiteparticles of the present invention, were preferable because thedischarge capacity was large and the irreversible capacity wasparticularly small.

TABLE 8 Comparative Example 3a Example 3a Graphite particles used innegative electrode Ex. Ex. Ex. Ex. Ex. CE CE CE CE 1a 1d 1e 1f 1g 1a 1b1c 1d Discharge capacity 362 365 367 364 363 365 321 330 345 (mAh/g)Irreversible 28 26 27 27 28 55 30 60 55 capacity (mAh/g) [CE:Comparative Example]

Example 3b

The graphite particles of Examples 1h to 1k and the raw materialgraphite particles of Comparative Example 1e were evaluated when used ina lithium ion secondary cell negative electrode under the conditionsgiven in Table 9. These results are given in Table 10.

TABLE 9 Category Conditions Cell 2-pole (counter electrode: metalliclithium) Sample weight 8 mg Electrode surface area 2.5 cm² Binderpolyvinylidene fluoride (#1120 made by Kureha), 10 wt % Solvent used toprepare N-methyl-2-pyrrolidone electrode mix paste Drying conditions110° C., 5 hours, in the atmosphere Electrolyte 1 M LiPF₆ ethylenecarbonate/methyl ethyl carbonate (1/1) Charging conditions constantcurrent charging 0.2 mA constant voltage charging 0 V, 0.02 mADischarging conditions current 0.2 mA cutoff voltage 1.5 V

TABLE 10 Charging and discharging characteristics (1^(st) cycle) Ex. 1hEx. 1i Ex. 1j Ex. 1k CE 1e Charge capacity (mAh/g) 400 398 396 395 394Discharge capacity 371 368 366 363 359 (mAh/g) Irreversible capacity 2930 30 32 35 (mAh/g) Cycling efficiency (%) 92.8 92.5 92.4 91.9 91.1 [CE:Comparative Example]

Example 3c

The graphite particles of Examples 1o to 1t and Comparative Examples 1fand 1 g were evaluated when used in a lithium ion secondary cellnegative electrode under the conditions given in Table 9. These resultsare given in

TABLE 11 Charging and discharging characteristics (1^(st) cycle) Ex. Ex.Ex. Ex. Ex. Ex. CE CE 1o 1p 1q 1r 1s 1t 1f 1g Charge 394 395 393 393 397395 399 394 capacity (mAh/g) Discharge 363 365 361 362 362 363 362 359capacity (mAh/g) Irreversible 31 30 32 31 33 32 37 35 capacity (mAh/g)Cycling 92.1 92.4 91.9 92.1 91.2 91.9 90.7 91.1 efficiency (%) [CE:Comparative Example]

Lithium Secondary Cell Having Nonaqueous Electrolyte Secondary CellNegative Electrode Made From Graphite Particles Example 4a

The lithium secondary cell of the present invention was produced asfollows, using the artificial graphite particles of the presentinvention produced in Examples 1a to 1g as the negative electrode activematerial.

The artificial graphite particles of the present invention were combinedwith polyvinylidene fluoride (PVDF; used as a binder) in proportions(weight ratio) of 90% and 10%, respectively, and N-methyl-2-pyrrolidone(NMP) was added as a solvent to prepare a negative electrode mix paste.One side of a copper foil with a thickness of 10 μm was coated with thisnegative electrode mix paste in an amount of 11 mg/cm² per unit ofsurface area, with this coating applied intermittently, leaving uncoatedportions at regular intervals. After this, the negative electrode mixpaste was dried to remove the NMP and form a negative electrode mixfilm. A negative electrode mix film was similarly formed on the otherside of the copper foil, which yielded a coated electrode. Themix-coated and uncoated portions on both sides of the copper foil weremade to match up with each other in this coating. After this, the coatedelectrode was press molded in a roll press until the density of thenegative electrode mix was 1.5 g/cm³, which produced a negativeelectrode sheet. This negative electrode sheet was cut into a longstrip, so that the end of the cut negative electrode sheet was uncoatedcopper foil. A negative electrode tab was attached to this copper foilportion by ultrasonic welding.

Meanwhile, a positive electrode mix paste was prepared by using anactive material expressed by the chemical formula LiCoO₂ for thepositive electrode, combining this positive electrode active materialwith polyvinylidene fluoride (PVDF; used as a binder) and carbon black(used as a conducting aid) in proportions (weight ratio) of 90%, 5%, and5%, respectively, and adding N-methyl-2-pyrrolidone (NMP) as a solvent.One side of an aluminum foil with a thickness of 20 μm was coated withthis positive electrode mix paste in an amount of 24 mg/cm² per unit ofsurface area, with this coating applied intermittently, leaving uncoatedportions at regular intervals. After this, the positive electrode mixpaste coating was dried to remove the NMP and form a positive electrodemix film. A positive electrode mix film was similarly formed on theother side of the aluminum foil, which yielded a coated electrode. Themix-coated and uncoated portions on both sides of the aluminum foil weremade to match up with each other in this coating. After this, the coatedelectrode was press molded in a roll press until the density of thepositive electrode mix was 3.3 g/cm³, which produced a positiveelectrode sheet. This positive electrode sheet was cut into a longstrip, so that the end of the cut positive electrode sheet was uncoatedaluminum foil. A positive electrode tab was attached to this aluminumfoil portion by ultrasonic welding.

The above-mentioned negative and positive electrodes and a separatorconsisting of a microporous film of polyethylene were laminated in theorder of negative electrode, separator, positive electrode, andseparator, and these were coiled to form an electrode group. Thepositive electrode tab and the negative electrode tab were the top andbottom of the coiled group, respectively. This electrode group wasinstalled in a cell can, and the positive electrode tab was connected byspot welding to the cell inner lid, and the negative electrode tab tothe cell can.

The electrolyte was a solution obtained by dissolving LiPF₆ in an amountof 1 mol/dm³ in a mixed solvent of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) with a volumetric ratio of 1:1.

The electrolyte was poured into the cell can, after which the cell lidwas attached to the cell can to produce the AA-size cylindrical lithiumsecondary cell of the present invention.

Comparative Example 4a

An AA-size cylindrical lithium secondary cell was produced in the samemanner as in Example 4a, but using the conventional graphite particlesof Comparative Examples 1a to 1d.

A charging and discharging test was conducted using the lithiumsecondary cells produced in Example 4a and Comparative Example 4a.Initial charging was conducted at a constant current of 120 mA(equivalent to 0.2 C), then charging was performed at a constant voltageof 4.2 V after the cell voltage reached 4.2 V, which was concluded atthe point when the charging time reached 7 hours. Meanwhile, initialdischarging was performed at a constant current of 120 mA (equivalent to0.2 C), and was concluded at the point when the cell voltage reached 3.0V.

Charging and discharging were performed for three cycles under the aboveconditions, after which the discharge current was changed to 600 mA(equivalent to 1 C) and 1200 mA (equivalent to 2 C) on the fourth andfifth cycles, and the load characteristics were examined.

Further, the cycle of constant current and constant voltage chargingfollowed by constant current discharging was repeated as above, with thecharging current set to 600 mA (equivalent to 1 C), the end time to 2.5hours, and the discharging current to 600 mA (equivalent to 1 C).

Tables 12 and 13 show the initial discharge capacity of the lithiumsecondary cells produced in Example 4a and Comparative Example 4a, thecoulomb efficiency in initial charging and discharging, the capacityretention at 1 C and 2 C discharge versus 0.2 C discharge, and thecapacity retention after 300 cycles versus initial discharge. It can beseen that the lithium secondary cells of the present invention exhibitedlarger discharge capacity, higher initial coulomb efficiency, andsuperior load characteristics and cycling characteristics as compared tothe conventional lithium secondary cells.

TABLE 12 Example 4a Graphite particles used for negative electrode Ex.1a Ex. 1d Ex. 1e Ex. 1f Ex. 1g Initial discharging 621 624 627 629 629capacity (mAh/g) Coulomb efficiency in 92.7 93.2 93.1 93.1 92.5 initialcharging (%) Capacity retention in 1 C 98.3 98.3 98.5 98.6 98.6discharge versus 0.2 C discharge (%) Capacity retention in 2 C 93.7 94.895.2 95.0 95.3 discharge versus 0.2 C discharge (%) Capacity retentionafter 89.3 91.5 88.7 90.1 90.5 300 cycles versus initial discharge (%)

TABLE 13 Graphite particles used Comparative Example 4a for negativeelectrode CE 1a CE 1b CE 1c CE 1d Initial discharging 581 545 512 553capacity (mAh/g) Coulomb efficiency in 86.6 91.0 84.0 86.2 initialcharging (%) Capacity retention in 1 C 94.2 93.8 95.2 93.2 dischargeversus 0.2 C discharge (%) Capacity retention in 2 C 87.2 85.3 84.9 87.9discharge versus 0.2 C discharge (%) Capacity retention after 81.1 82.375.2 78.6 300 cycles versus initial discharge (%) [CE: ComparativeExample]

Further, using the lithium secondary cells produced in Example 4a andComparative Example 4a, charge receptability was evaluated according tothe capacity that can be achieved in constant current charging until thecharging voltage reached 4.2 V at a maximum current of 1200 mA(equivalent to 2 C). Table 14 shows the quick charging capacity incharging by the method described above. It can be seen from Table 14that the lithium secondary cells of the present invention exhibitedlarger charging capacity and superior charging receptability as comparedto the conventional lithium secondary cells.

TABLE 14 Example 4a Comparative Ex. 4a Graphite particles used fornegative electrode Ex. Ex. Ex. Ex. Ex. CE CE CE CE 1a 1d 1e 1f 1g 1a 1b1c 1d Quick charging 313 341 338 295 289 153 185 120 137 capacity(mAh/g) [CE: Comparative Example]

Example 4b

The lithium deposition state on the negative electrode surface afterovercharging was examined by the following method for the lithiumsecondary cell of the present invention produced using the negativeelectrode material of Example 1a in Example 4a and for the conventionallithium secondary cell produced using the artificial graphite ofComparative Example 1a in Comparative Example 4a. Using theabove-mentioned lithium secondary cell of the present invention andconventional lithium secondary cell, first constant current charging wasperformed at 120 mA (equivalent to 0.2 C), then charging was performedat a constant voltage of 4.2 V from the point when the cell voltagereached 4.2 V, which was concluded at the point when the charging timereached 7 hours. Next, a state of overcharge was achieved by chargingfor 5 minutes at a constant current of 1200 mA (equivalent to 2 C).These cells were disassembled and their negative electrodes removed, andthe state of lithium deposition on the negative electrode surface wasexamined by scanning electron micrograph (SEM).

FIG. 12 shows the state of the negative electrode of the lithiumsecondary cell of the present invention in this overcharged state, whileFIG. 13 shows the state of the negative electrode in the conventionallithium secondary cell. For the sake of reference, FIGS. 14 and 15 showunused electrodes of each, on which no lithium has been deposited. Acomparison of the SEM images before and after overcharging revealsdeposited lithium on the negative electrode in the overcharged state,and it can be seen in the SEM image in FIG. 12 that lithium wasdeposited in the form of particles on the negative electrode of thelithium secondary cell of the present invention, and these particlesstuck together to give the appearance of moss, whereas the lithium wasdeposited in dendritic form on the negative electrode of theconventional lithium secondary cell. Thus, it can be seen that dendriticdeposition of lithium is suppressed when the artificial graphiteparticles of the present invention are used for the negative electrodematerial.

INDUSTRIAL APPLICABILITY

As described above, the present invention provides artificial graphiteparticles and a method for manufacturing the same, with which theelectrolyte decomposition reaction in initial charging is suppressed andirreversible capacity is reduced, without sacrificing the advantage of agraphite negative electrode of being capable of occluding and releasinga large amount of lithium, thereby increasing the capacity of a lithiumsecondary cell, and provides a nonaqueous electrolyte secondary cellnegative electrode in which these artificial graphite particles areused, and a method for manufacturing this electrode, as well as alithium secondary cell that makes use of this nonaqueouselectrolyte-secondary cell negative electrode.

The lithium secondary cell of the present invention, in which theartificial graphite particles of the present invention are used as alithium secondary cell negative electrode material, has large capacity,excellent quick charging and discharging characteristics, and verylittle cycling deterioration. Furthermore, the artificial graphiteparticles of the present invention undergo little electrolytedecomposition, and the irreversible capacity is small, electrodecoatability and electrode adhesion are excellent, which makes possiblethe above-mentioned improvements in a lithium secondary cell. Also, themethod for manufacturing artificial graphite particles of the presentinvention comprises a simple manufacturing process and allows a largequantity of artificial graphite particles to be manufactured.

Finally, coatability and adhesion of the negative electrode activematerial are improved, and quick charging and dischargingcharacteristics and cycling characteristics can be enhanced with thepresent invention, so the present invention provides a lithium secondarycell that is favorable for cellular telephones, notebook personalcomputers, and other such portable devices, electric automobiles, powerstorage devices, and so forth.

1. Artificial graphite particles having secondary particles, thesecondary particles comprising a plurality of primary particles, whereinthe primary particles are composed of graphite and bonded together toform the secondary particles; wherein edge portions of the primaryparticles are bent in a polyhedral shape; wherein the edge portions ofthe primary particles in polyhedral shape are disposed only on anoutermost surface of the secondary particles, which allow edge portionsof the secondary particles forming the artificial graphite particles tobe bent in a polyhedral shape; and wherein a specific surface area ofthe artificial graphite particles is 3 to 6 m²/g.
 2. The artificialgraphite particles according to claim 1, wherein the outermost surfaceof the secondary particles has a surface layer in a low-crystallinity oramorphous state.
 3. The artificial graphite particles according to claim1, wherein the secondary particle structure is one in which a pluralityof primary particles composed of graphite are bonded together in anon-parallel manner, and the structure has voids inside the secondaryparticles.
 4. The artificial graphite particles according to claim 1,wherein the bulk density is at least 0.8 g/cm³ and the surface oxygenconcentration as measured by x-ray photon spectrometry (XPS) is 1.0 to4.0 atom %.
 5. The artificial graphite particles according to claim 4,wherein the surface oxygen concentration is 1.0 to 3.0 atom %.
 6. Theartificial graphite particles according to claim 1, wherein the peakintensity ration (R=I₁₃₆₀/I₁₅₈₀) between the peak (I₁₃₆₀) appearing at1360 cm⁻¹ and the peak (I₁₅₈₀) appearing at 1580 cm⁻¹ in Raman spectrummeasurement is 0.1 to 0.5 (0.1≦R≦0.5).
 7. The artificial graphiteparticles according to claim 6, wherein the peak intensity ratio(R=I₁₃₆₀/I₁₅₈₀) is 0.1 to 0.4 (0.1≦R≦0.4).
 8. The artificial graphiteparticles according to claim 1, wherein weight reduction and heatgeneration occur at a temperature of at least 640° C., and the weightreduction caused by heating for 30 minutes at 650° is less than 3%, inthermogravimetric-differential thermal analysis (TG-DTA) under an airflow.
 9. The artificial graphite particles according to claim 1, whereinthe pore volume measured by mercury porosimetry is 0.1 to 0.5 cm³/g. 10.The artificial graphite particles according to claim 1, wherein thespacing d002 of the (002) plane in the graphite is less than 0.337 nm,the average particle size is 10 to 50 μm, and the true density is atleast 2.2 g/cm³.
 11. The artificial graphite particles according toclaim 1, wherein an electrode mix paste obtained by kneading theartificial graphite particles with binder (a) and solvent (b) has aviscosity of 0.3 to 1.6 Pa·s at a temperature of 25° C. and a shear rateof 4 sec⁻¹; (a) polyvinylidene fluoride, where the weight ratio of thebinder to the artificial graphite particles is 1:9, and thepolyvinylidene fluoride is a polyvinylidene fluoride that exhibits asolution viscosity of 550±100 mPa·s when 12.0±0.5 wt %N-methyl-2-pyrrolidone solution is produced, (b) N-methyl-2-pyrrolidone,in a weight of 45% with respect to the total amount of the paste. 12.The artificial graphite particles according to claim 11, wherein theviscosity is 1.0 to 1.3 Pa·s.
 13. The artificial graphite particlesaccording to claim 1, wherein an electrode mix paste has a shear ratedependence (TI) of viscosity at 25° C. of 2.0 to 4.0, the paste beingobtained by kneading the artificial graphite particles with binder (a)and solvent (b) and the paste having a viscosity of 1.0 Pa·s at 25° C.and a shear rate of 4 sec⁻¹, where TI is defined by the formulaTI=(viscosity at shear rate of 4 sec⁻¹)/(viscosity at shear rate of 40sec⁻¹): (a) polyvinylidene fluoride, where the weight ratio of thebinder to the artificial graphite particles is 1:9, and thepolyvinylidene fluoride is a polyvinylidene fluoride that exhibits asolution viscosity of 550±100 mPa·s when 12.0±0.5 wt %N-methyl-2-pyrrolidone solution is produced, (b) N-methyl-2-pyrrolidone.14. The artificial graphite particles according to claim 13, wherein theshear rate dependence (TI) is 2.0 to 3.5.
 15. The artificial graphiteparticles according to claim 13, wherein the shear rate dependence (TI)is 2.6 to 3.0.
 16. A nonaqueous electrolyte secondary cell negativeelectrode having graphite particles that occlude or release alkali metalions, affixed by an organic binder to a metal foil surface, wherein thegraphite particles are composed of the artificial graphite particlesaccording to claim
 1. 17. A lithium secondary cell having a laminate,produced by the successive lamination of a negative electrode capable ofoccluding and releasing lithium, a separator, and a positive electrodecapable of occluding and releasing lithium, and a nonaqueous electrolytein a container, wherein the negative electrodes comprise the negativeelectrode according to claim
 16. 18. The lithium secondary cellaccording to claim 17, wherein the lithium occluded on the surface ofthe graphite particles is deposited in the form of particles or moss.19. The artificial graphite particles according to claim 2, wherein thesecondary particle structure is one in which a plurality of primaryparticles composed of graphite are clustered or bonded together in anon-parallel manner, and the structure has voids inside the secondaryparticles.